When a moving image is digitized and transmitted via a network represented by the Internet etc., an amount of data thereof is enormous. It is generally performed that the moving image is coded to reduce the amount of data.
At this time, as methods for controlling the amount of coded data, there are fixed bit rate coding and variable bit rate coding. The fixed bit rate coding has such a system that, as shown in FIG. 17, a buffer 1002 is provided between a coding apparatus 1001 and a transmission path and fluctuation of a code amount in a short term is absorbed as well as control is performed so that the amount of generated codes matches a transmission bit rate such as by applying feedback controlling based on the amount of generated codes in the past and remaining capacity of the buffer. Note that, one even including the buffer is sometimes called the coding apparatus.
For example, in Patent Literature 1 described below, when a scene whose coding difficulty is high is coded continuously, a method for setting a target bit rate of a scene whose coding difficulty is low to be small is taken to secure the target bit rate. FIG. 18 shows an example of rate control by coding difficulty, where (a) shows a relation between coding difficulty and time, (b) shows a relation between a target bit rate and time and (c) shows a relation between a code amount and time. Specifically, in FIG. 18, since the coding difficulty of a scene in a term Tc is high, target bit rates for scenes in terms Ta and Tb that are just before the scene and whose coding difficulty is low are set to be small to suppress an amount of generated codes to be small and increase the remaining capacity of a buffer. Then, with the use of the remaining capacity of the buffer, the target bit rate for the scene in the term Tc is set to be high so that probability that deterioration of image quality is generated is reduced in the scene that is difficult to code.
On the other hand, the variable bit rate coding has a system that is usable for transmission in a high-speed packet network, which has been studied in ATM (asynchronous transfer mode) video coding etc. (Non-Patent Literature 1 described below). In ideal variable bit rate coding, only necessary information may be coded and there is an advantage that constant image quality can be kept.
Meanwhile, hierarchical coding is to transmit video signal information by dividing into information of plural hierarchies. Here, a hierarchical coding system as shown in FIG. 19 is examined and coded data is transmitted through a plurality of transmission paths. The hierarchization is carried out on the basis of such as from low quality to high quality, from a low rate to a high rate and from low resolution to high resolution. The hierarchical coding system shown in FIG. 19 is called a feedback type, in which a difference between a video signal and an input video signal transmitted in a low hierarchy is sequentially coded in a higher hierarchy.
That is, coding apparatuses 1100 for coding input video signals in each hierarchy are included and the signals coded by them are transmitted to decoding apparatuses 1300 via a transmission network 1200 in each transmission channel. The decoding apparatus 1300 decodes and reproduces the received signal. Here, the coding apparatus 1100 is comprised of a coder 1101 for coding an input video signal and a local decoder 1102 for decoding a coded signal, in which a difference between the video signal decoded by the local decoder 1102 and the input video signal to a low hierarchy serves as an input video signal to a next hierarchy.
For example, in Patent Literature 2 described below, a case of two hierarchies is described. Here, coded data is separated into a significant portion in terms of image quality (MSP) and an else portion (LSP), and packetizing is carried out using only the MSP or the LSP to transmit to a transmission path together with an identifier indicating whether the MSP or the LSP is included. Then, the MSP is sent in a high-priority transmission class and the LSP is sent in a low-priority transmission class. In the case of network congestion, discarding is performed from the packet including the LSP so as to correspond to packet loss which becomes a problem in ATM transmission.    Patent Literature 1: Japanese Patent Application Laid-open No. 2001-69463    Patent Literature 2: Japanese Patent Application Laid-open No. 1992-111695    Non-Patent Literature 1: Wada Masahiro, 1991 Picture Coding Symposium PCSJ91, pp 239-242